1. Technical Field
The present invention relates to an interference filter, an optical module, and an electronic apparatus.
2. Related Art
In the past, there has been known a variable wavelength interference filter used as an interference filter capable of causing multiple interference in the light between a pair of optical reflection films to thereby emit the light with a desired wavelength (see, e.g., JP-A-2008-76749 (Document 1)).
The variable wavelength interference filter described in Document 1 has substrates disposed so as to be opposed to each other, and the optical reflection films are respectively disposed on the surfaces of the respective substrates, the surfaces being opposed to each other. Further, the distance between the optical reflection films can be controlled, and it becomes possible to take out the light having the wavelength corresponding to the gap dimension between the optical reflection films from the incident light.
For the optical reflection film (hereinafter also referred to simply as an optical film) in such a variable wavelength interference filter, metal films and dielectric multilayer films can be used. It is preferable for the optical film to have both of superior reflectance characteristics and transmissivity in the wavelength range of the light used therein, and taking this condition into consideration, silver (Ag) with small film thickness could be a strong candidate of the metal film. However, silver has low heat resistance, and is easily sulfurized, and thus there is a concern about the low long-term reliability of silver.
Here, there has been proposed a technology of using an alloy thin film consisting primarily of silver for the optical film to thereby improve the reliability (see, e.g., JP-A-2009-251105 (Document 2)).
In Document 2, the optical film is formed of an alloy film of silver including carbon.
Further, an accumulation of static electricity may sometimes occur in the optical film in the variable wavelength interference filter, and in some cases, the control of the gap dimension between the optical films becomes difficult due to the accumulation of static electricity.
Here, there has been proposed a technology of providing a carbon film on the surface of the optical film to thereby improve the abrasion resistance and the antistatic property (see, e.g., JP-A-2006-84994 (Document 3)).
A neutral density (ND) filter of Document 3 used for an imaging system such as a camera or a video camera has a configuration of alternately stacking SiO2 and Ti metal compound, and further stacking a carbon film superior in the abrasion resistance and the antistatic property as the outermost layer thereof.
Regarding the variable wavelength interference filter of Document 1, the optical film is exposed to various chemicals or gasses in the manufacturing process performed after the optical film is formed, and is therefore easy to be deteriorated. In particular, in the case of using silver for the optical film, the deterioration is easy to occur.
In order to cope with the problem, according to Document 2, the optical films in the interference filter are formed of alloy films including carbon. By alloying silver, the resistance of silver to sulfurization and so on can be improved. However, in the long-term reliability, there is a concern about deterioration in characteristics. Therefore, it is difficult to solve the problem of the deterioration in the characteristics of the optical film only by the alloying, and therefore, some measures for preventing the deterioration in the characteristics of the optical film become necessary.
Further, although in Document 3 it is possible to prevent the accumulation of static electricity to some extent by incorporating carbon into the outermost layer of the ND filter, since the carbon film is in an electrically floating state, the complete elimination of the accumulation of static electricity is not possible. Further, although the upper surface of the ND filter is covered by the carbon film, the end surfaces (the side surfaces) thereof are exposed. Therefore, it results that the end surfaces are exposed to various chemicals or gasses in the manufacturing process, and the filter might be deteriorated.